docs: def new_feature_container(self, idx, path=None):
          """
          Add a new feature container with the given data.
  
          Parameters:
              idx (str): An unique identifier within the dataset.
              path (str): The path to store the feature file. If None a default path is used.
  
          Returns:
              FeatureContainer: The newly added feature-container.
          """
  
          new_feature_idx = idx
          new_feature_path = path
  
          # Add index to idx if already existing
          if new_feature_idx in self._feature_containers.keys():
              new_feature_idx = naming.index_name_if_in_list(new_feature_idx,
                                                             self._feature_containers.keys())
  
          # Set default path if none given
          if new_feature_path is None:
              if not os.path.isdir(self.path):
                  raise ValueError('To copy file the dataset needs to have a path.')
  
              new_feature_path = os.path.join(self.path, DEFAULT_FEAT_SUBDIR, new_feature_idx)
          else:
              new_feature_path = os.path.abspath(new_feature_path)
  
          feat_container = containers.FeatureContainer(new_feature_path)
          self._feature_containers[new_feature_idx] = feat_container
  
          return feat_container